Continuous casting machines (CCM) for slab casting are very large and complex production units. The height of the machines is approximately 20 meters (bow radius of around 10 meters) and 30 meters in length. The strand of the CCM consists in general of approximately 200 roll lines (400-600 rolls depending on one- or two-split design) with varying diameter (140-320 mm in diameter) that are cooled both internally and externally. A schematic picture of the strand can be seen in FIG. 1.
The mold is water cooled internally to solidify the liquid metal directly in contact with it. This cooling is referred to as primary cooling, see FIG. 1. The surface of the slab is approximately 1500° C. just below the mold at the foot rolls and decreases continuously to around 800-900° C. at the horizontal part of the strand. The cooling of the slab (and the rollers) is divided into internal cooling and external spray water cooling (secondary cooling). The top rolls (foot rolls) are only water cooled externally, while the rest of the rolls are water cooled internally by e.g. using center bore design, revolver design or spiral bore design.
The entire slab is spray cooled externally with air mist or water. The nozzles may be placed in such a way that the water sprays between the rolls in order to maximize the cooling of the slab. The combination of very high temperatures and a wet and humid environment in the strand results in extremely harsh conditions for the casting mill equipment in general and for the caster rolls in particular. The low alloyed caster rolls are therefore coated with stainless steel using hardfacing (for example weld or laser cladding). This will dramatically improve the caster roll surface resistance to failure mechanisms and thereby increase the total service life time of the CCM.
There are two types of roll failure mechanisms, catastrophic failure and surface deterioration. Catastrophic failure includes neck breakages, cracks through the body or melted bodies due to break-outs in the strand. Breakages or cracks are very rare as the core bodies of today are shorter (one or two split caster roll lines) compared to earlier generation full-body caster rolls. Break-outs are also less common today due to advanced modern production control systems.
Regarding surface deterioration, the service conditions of the caster rolls (also known as roll mantles) are complex and vary dependent on the roll location within the strand and the roll line. However, it is well known that the most exposed roll line location is where the slab changes its direction from vertical to horizontal. The middle position of the roll line is also more prone to deterioration compared to the flank positions. There are many different failure mechanisms responsible for the surface deterioration and they can be summarized in the areas of corrosion, wear and fatigue.